Data communications for video, and so forth have recently undergone a tremendous explosion in use, so that network-capacity crunch and frequency-resources crunch are anticipated on a worldwide basis, posing a major problem. In order to cope with this situation, development of a microwave filter for use by a base station of mobile communications is asked for so as to concurrently implement the effective utilization of high-speed/large-capacity communication and frequency-resources. Further, communication equipment capable of coping with a plurality of frequency-bands is also lately desired.
A method of communication by the concurrent use of two frequency-bands has been proposed as one of the methods for implementing high-speed/large-capacity communication. There has been proposed a dual-band band-pass filter which allows two frequency-bands to concurrently pass therethrough, representing the elemental technology of the method described as above.
There exists the traditional method of making up a dual-band band-pass filter having two pass-bands, as described hereunder. As shown in FIG. 1, a plurality of a dual-band resonators N1, N2, and N3, resonating at two frequencies, are subserviently coupled with each other, to be coupled with input/output ports M1, M2, provided at the respective ends of subservient coupling, thereby making up a filter 100 (Non-patent Document 1).
The dual-band resonators N1, N2, N3 each has even-number/odd-number modes, and the dual-band band-pass filter having the two pass-bands is made up by controlling these two modes. With this filter 100, there is the need for directly coupling the input/output ports M1, M2 to the dual-band resonators N1, and N3, at the respective ends of the filter 100, thereby deciding a connection position for enabling desired characteristics to be concurrently obtained in both the two pass-bands.
Meanwhile, a band-pass filter having an abrupt cut-off characteristic is required for effective utilization of frequency-resources. In general, an abrupt cut-off characteristic can be realized by adoption of a multistage operation to thereby increase the number of resonators. However, since a normal conductor, such as copper, etc., has a resistance, an insertion loss will increase along with the multistage operation, so that it has been impossible to realize a trade-off between a low-loss and the abrupt cut-off characteristic. There has been proposed, for example, a superconducting band-pass filter, as one of the methods for solving this problem (Patent document 1). As a superconductor has a surface resistance lower by 2 to 3 orders of magnitude as compared with copper, etc., in a microwave-band, so that the insertion loss can be suppressed to be low even after a multistage operation, thereby enabling the trade-off between the low-loss and the abrupt cut-off characteristic to be realized.
Further, there has been proposed a center-frequency tunable dual-band band-pass filter capable of varying the center-frequency of the band-pass filter, as a method coping with a plurality of frequency-bands.
FIG. 2 of Patent document 2 is a view showing the configuration of a traditional superconducting tunable dual-band band-pass filter by way of example. A dielectric plate S10 is disposed above a microstrip-type filter pattern S1 formed on a dielectric substrate S5. A distance h between the dielectric plate S10 and the filter pattern S1 is varied by use of an actuator such as a piezoelectric element, etc. and so forth, to cause an electric-field distribution radiated from the filter pattern S1 to be varied, thereby rendering the center-frequency variable (Patent document 2).
A dual-band band-pass filter is realized by use of a dual-band resonator capable of realizing a resonance frequency in two pass-bands by use of one resonator. A center-frequency, in the respective bands of the dual-band band-pass filter shown in FIG. 1, is dependent on the even-number/odd-number modes occurring to the respective dual-band resonators N1, N2, and N3. Because the respective odd-number mode portions of the dual-band resonators N1, N2, N3 are for use in common with the even-number modes, adjustment of the odd-number modes affects the even-number modes. A bandwidth of each band is controlled by a distance between the resonators with respect to the respective dual-band resonators N1, N2, N3, however, if the tuning of the center-frequency is applied to this dual-band band-pass filter by use of the dielectric plate S10, this will cause the whole surface above the dual-band band-pass filter to be covered with the dielectric plate. Accordingly, both modes of the even-number/odd-number modes of the dual-band band-pass filter will be affected, so that the tuning of the center-frequency is possible, however, it is difficult to tune the center-frequency independently of the even-number/odd-number modes. Further, since the filter in whole is covered with the dielectric plate, an electromagnetic field distribution between the dual-band resonators, as well, will be affected, thereby causing a problem in that the bandwidth as well undergoes variation. Furthermore, an increase in the shift amount of the center-frequency will result in degeneration of the band-pass characteristics, so that a trimming mechanism for adjustment of the resonance frequency of each of the resonators will be required aside from a frequency tuning mechanism. Herein, by “trimming” is meant a method of improvement of the band-pass characteristics.
The inventors have proposed a center-frequency tunable dual-band band-pass filter, as a method of solving these problems, as shown in Patent Document 3. The center-frequency tunable dual-band band-pass filter is capable of tuning (varying) two pass-bands independently of respective center-frequencies, and improving the band-pass characteristics that will degenerate after the tuning of the respective center-frequencies by use of the frequency tuning mechanism without newly introducing the trimming mechanism.
Further, in FIG. 3, mention is made of a dielectric rod 25 that has an ellipse-like cross section and is highly efficient in varying a shift amount of resonance in the odd-number mode, thereby proposing that the shift amount of the resonance in the odd-number mode can be changed by installing the dielectric rod 25, in a space above a half-wavelength resonator 10. However, upon fine-tuning of the space between the half-wavelength resonator 10 and the dielectric rod 25, if the cross section of the dielectric rod 25 is ellipse-like in shape, a tip portion of the long diameter of the ellipse will exceed the width of the strip conductor of the half-wavelength resonator 10 when rotated, thereby causing a characteristic in value to be extremely changed, so that there is the need for causing the dielectric rod 25 to approach the half-wavelength resonator 10, or to distance itself from the half-wavelength resonator 10. Accordingly, upon the fine-tuning of the space between the half-wavelength resonator 10 and the dielectric rod 25, a special device is required of the dielectric rod 25 having an ellipse-like cross section.
In the case of a dual-band band-pass filter using a superconductor, in particular, a measurement is conducted in a vacuum chamber, while cooling to −200° C. or lower by use of a refrigerator, and there is therefore the need for raising or lowering the dielectric rod 25 by vertically moving it from outside the chamber. For this reason, it is particularly desirous for the dielectric rod 25 to be structured so as to be pushed in by turning a screw. In such a case, if the rod 25 is formed in the shape of an ellipse in order to increase the shift amount of the resonance in the odd-number mode, there will arise the need for a mechanism capable of raising and lowering the rod 25 without rotating the same, whereupon the tuning mechanism will be considerably complicated to thereby cause an increase in size. In this case, there is a possibility that the tuning mechanism itself cannot be installed above the filter, thereby posing a problem in that use of the rod 25 in the shape of the ellipse is not practical from the viewpoint of cost.